vt: do not update when the console is blanked

vt.c DO_UPDATE macro checks if the console is visible but doesn't check if
the console is blanked.

In fact updating fbcon while the console is blanked is not only
unnecessary but can even cause screen corruption.

Therefore I am adding a simple check on console_blanked in DO_UPDATE.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/vt.c

@@ -261,7 +261,7 @@ static void notify_update(struct vc_data *vc)
 #ifdef VT_BUF_VRAM_ONLY
 #define DO_UPDATE(vc)	0
 #else
-#define DO_UPDATE(vc)	CON_IS_VISIBLE(vc)
+#define DO_UPDATE(vc)	(CON_IS_VISIBLE(vc) && !console_blanked)
 #endif
 
 static inline unsigned short *screenpos(struct vc_data *vc, int offset, int viewed)